Neon RT 2001 problems

Hi I hope some can help I have a fair few questions relating to 2001 Neon RT.
The Car in general is brilliant but we have recently noticed some issues[LIST][*]Tracking control turns itself off when vehicle turns[*]cuts out (infrequently) whilst idele or when powering

We have found the error codes:
P1684 , P0340, P0551

We are aware of the steering pressure problem, but can someone give us some advice on the other codes (ie how to locate and fix them) also where to get the relevant parts in the UK as we also need to get new headlamps and on dash/wheel light switch and having problems finding parts.

Thanks all,
The rockauto site seems perfect thankyou, however has anyone got any suggestions for decent headlamps, either original or styling mods not fussed as the originals on the car have been scratched badly